Appratus for measuring ground leakage current in an ungrounded direct current power system, and method for same

ABSTRACT

Provided a ground leakage current measurement apparatus in an ungrounded DC power system including main and auxiliary electric lines includes: a switching unit configured to perform switching to supply measurement power to a main ground resistor and an auxiliary ground resistor by using power of the electric lines; a measurement unit connected between the switching unit and the ground and configured to measure at least one of main and auxiliary ground leakage currents; and a control unit configured to control the switching unit to discriminate a main ground leakage current operation and an auxiliary ground leakage current operation of the measurement unit.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of PCT/KR 2010/001457 filed on Mar.9, 2010, which claims priority of Korean patent application number10-2009-0021958 filed on Mar. 16, 2009. The disclosure of each of theforegoing applications is incorporated herein by reference in itsentirety.

BACKGROUND OF THE INVENTION

The present invention relates to technology for measuring a groundleakage current in an ungrounded DC (Direct Current) power system, andmore particularly, to a ground leakage current measurement apparatus andmethod, which is capable of smoothly measuring ground leakage currentsat both sides although a ground fault simultaneously occurs in main andauxiliary electric lines in a live wire state or ground faults havingthe same ground resistance simultaneously occur.

In an ungrounded DC power system, an apparatus for measuring a groundleakage current of an electric line in a live wire state is mainly usedfor alarming a ground fault and installed in a main power booth.Furthermore, a ground resistance measuring instrument may be installedin the main power booth.

In general, a ground fault alarm circuit employs a voltage dividercircuit system using two resistors R1 and R2 as shown in FIG. 1(hereinafter, referred to as ‘2R system’), and a ground resistancemeasuring circuit employs a system for generating a measurement voltageby using two Zener diodes ZD1 and ZD2 as shown in FIG. 2 (hereinafter,referred to as ‘2ZD system’).

Both systems will be described more specifically as follows. First, acase in which ground faults having the same ground resistance valueoccur in both electric lines may be considered in the 2R system. In thiscase, a divided voltage across both ground fault alarms V1 and V2 fordetecting a displacement voltage is not changed in the 2R system.Therefore, the ground fault alarm circuit cannot be operated.

Furthermore, a case in which ground faults having different groundresistance values occur in both electric lines may be considered in the2R system. In this case, since a ground potential is decided by parallelcombined resistance values R1//RG1 and R2//RG2 in the main and auxiliarysides, alarm operating points of the ground fault alarms V1 and V2 maydiffer from initial set values. Here, the alarm operating pointindicates a critical value which is preset on the assumption that aground fault occurs only in one electric line.

Furthermore, even when a ground fault occurs only in one electric linein the 2R system, it is impossible to measure a linear displacementvoltage depending on variations in ground resistance value. For example,when a main ground resistor RG1 varies from 10 kΩ to 20 kΩ in a state inwhich the resistors R1 and R2 are set to 1 kΩ in FIG. 1, a parallelcombined resistance value R//RG2 in the case of 10 kΩ is about 0.91 kΩ,and a parallel combined resistance value R1//RG2 in the case of 20 kΩ isabout 0.95 kΩ. That is, while the value of the ground resistor RG1doubles, the parallel combined resistance value varies about 1.04 times(0.95/0.91), which means that the variation is non-linear.

Meanwhile, a case in which ground faults having the same groundresistance value occur in both electric lines may be considered in the2ZD system. When it is assumed that a voltage between the electric linesis 48V, a reference voltage of the Zener diodes ZD1 and ZD2 is 20V, andthe ground resistors RG1 and RG2 have the same value, a voltage of 24Vis applied to each ground resistor. In this case, a voltage measured byground leakage current testers A1 and A2 is 4V. Therefore, the groundleakage current testers A1 and A2 measure a different value from anactual ground resistance value.

Furthermore, a case in which ground faults having different groundresistance values occur in both electric lines may be considered in the2ZD system. At this time, when it is assumed that a voltage between theelectric lines is 48V, a reference voltage of the Zener diodes ZD1 andZD2 is 20V, the ground resistor RG1 has a resistance value of 10 kΩ, andthe ground resistor RG2 has a resistance value of 20 kΩ, a voltageacross both ends of the ground resistor RG1 is 16V. Therefore, anactually measured voltage is reduced to 4V. Furthermore, since a voltageacross both ends of the ground resistor RG2 is 32V, an actually measuredvoltage becomes a value smaller than zero. Therefore, it is impossibleto measure a ground fault in the auxiliary side.

In the 2ZD system, even when a ground fault occurs only in one electricline, for example, in the main side (RG1=10 kΩ), a current flowing inthe ground leakage current tester A1 is 0.4 mA (=4V/10 kΩ). Here, sincethe reference voltage is 20V, the ground resistance value is measured at500 kΩ, not 10 kΩ.

Meanwhile, a distributed capacitor component exists between an electricline and the ground, and a capacitor component such as a noise removalcapacitor installed in a load exists. As well known, this capacitorcomponent acts as noise during the ground leakage current measurement,and may serve as a cause which makes it difficult to measure an accurateground resistance value. Furthermore, in order to detect a ground faultposition, a method in which a supervisory signal of a signal generatoris periodically transmitted to an electric line and a zero phasesequence current transformer (ZCT) installed in a diverging point of theelectric line detects a ground leakage current of the diverging pointbased on the supervisory signal is widely used. When the supervisorysignal transmitted to the electric line is a sine wave signal, acapacitive ground leakage current has a phase which leads the phase of aresistive ground leakage current by 90 degrees. When the supervisorysignal is a square wave signal, the capacitive ground-leakage currentexhibits a spike peak. That is, in order to more accurately measure aground leakage current, the influence of the signal generator as well asthe influence of the above-described capacitor components should beremoved.

SUMMARY OF THE INVENTION

An embodiment of the present invention is directed to a ground leakagecurrent measurement apparatus and method capable of measuring groundleakage current at both sides, even though ground faults simultaneouslyoccur in main and auxiliary electric lines.

Another embodiment of the present invention is directed to a groundleakage current measurement apparatus and method capable of measuring aground leakage current, even though ground faults having the same groundresistance value simultaneously occur in main and auxiliary electriclines.

Another embodiment of the present invention is directed to a groundleakage current measurement apparatus and method capable of minimizingthe influence of capacitive ground leakage current caused by a capacitorcomponent of a load and a distributed capacitor component between anelectric line and the ground.

Another embodiment of the present invention is directed to a groundleakage current measurement apparatus and method capable of detecting aground fault position without using a separate signal generator.

In accordance with an embodiment of the present invention, a groundleakage current measurement apparatus in a ungrounded DC power systemincluding main and auxiliary electric lines includes: a switching unitconfigured to perform switching to supply measurement power to a mainground resistor and an auxiliary ground resistor by using power of theelectric lines; a measurement unit connected between the switching unitand the ground and configured to measure at least one of main andauxiliary ground leakage currents; and a control unit configured tocontrol the switching unit to discriminate a main ground leakage currentoperation and an auxiliary ground leakage current operation of themeasurement unit.

The switching unit of the ground leakage current measurement apparatusmay include a main storage section configured to store main measurementpower from the power of the electric lines; a main switching sectionconfigured to perform switching to supply the main measurement power tothe main ground resistor; an auxiliary storage section configured tostore auxiliary measurement power from the power of the electric lines;and an auxiliary switching section configured to perform switching tosupply the auxiliary measurement power to the auxiliary ground resistor.

The control unit of the ground leakage current measurement apparatus maycontrol the main switching section to supply the main measurement powerto a load before the main ground leakage current measurement operationof the measurement unit, thereby charging a main capacitor component ofthe load.

The control unit of the ground leakage current measurement apparatus maycontrol the auxiliary switching section to supply the auxiliarymeasurement power to a load before the auxiliary ground leakage currentmeasurement operation of the measurement unit, thereby charging anauxiliary capacitor component of the load.

In accordance with another embodiment of the present invention, a groundleakage current measurement method in an ungrounded DC power systemincluding main and auxiliary electric lines and a control unit includes:supplying measurement power to a main or auxiliary ground resistor byusing power of the electric lines, under control of the control unit;and measuring at least one of main and auxiliary ground leakage currentspassing through the ground resistor, under control of the control unitwhich discriminates a measurement operation.

The supplying of the measurement power may include storing mainmeasurement power by using the power of the electric lines; andsupplying the main measurement power to the main ground resistor.

The supplying of the measurement power may include storing auxiliarymeasurement power by using the power of the electric lines; andsupplying the auxiliary measurement power to the auxiliary groundresistor.

The ground leakage current measurement method may further includesupplying the main measurement power to a load to charge a maincapacitor component of the load, before supplying the main measurementpower to the main ground resistor. Furthermore, the ground leakagecurrent measurement method may further include supplying the auxiliarymeasurement power to a load to charge an auxiliary capacitor componentof the load, before supplying the auxiliary measurement power to theauxiliary ground resistor.

ADVANTAGEOUS EFFECTS

In accordance with the embodiments of the present invention, the mainground leakage current measurement operation and the auxiliary groundleakage current measurement operation are exclusively mutuallyperformed. Therefore, although ground faults simultaneously occur in themain and auxiliary electric lines or ground faults having the sameground resistance value occur, it is possible to measure ground leakagecurrents in both sides.

Furthermore, before an actual ground leakage current measurementoperation is performed, the capacitor component of the load and thedistributed capacitor component between the electric line and the groundare charged. Therefore, the influences of capacitive ground leakagecurrent caused by the capacitor components are minimized. As such, sincea pure resistive ground leakage current caused by the ground resistor ismeasured, it is possible to increase the reliability.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating a schematic circuit configuration of aconventional 2R system.

FIG. 2 is a diagram illustrating a schematic circuit configuration of aconventional 2ZD system.

FIG. 3 is a schematic function block diagram of a ground leakage currentmeasurement apparatus in accordance with an embodiment of the presentinvention.

FIG. 4 is a diagram explaining a ground leakage current measurementoperation in accordance with the embodiment of the present invention.

FIG. 5 is a diagram illustrating a circuit configuration of the groundleakage current measurement apparatus in accordance with the embodimentof the present invention.

FIG. 6 is a flow chart explaining a ground leakage current measurementprocess in accordance with another embodiment of the present invention.

FIG. 7 is a diagram explaining an operation of storing main measurementpower in accordance with the embodiment of the present invention.

FIG. 8 is a diagram explaining an operation of charging a main capacitorcomponent of a load in accordance with the embodiment of the presentinvention.

FIG. 9 is a diagram explaining an operation of measuring a main groundleakage current in accordance with the embodiment of the presentinvention.

FIG. 10 is a diagram explaining an operation of storing auxiliarymeasurement power in accordance with the embodiment of the presentinvention.

FIG. 11 is a diagram explaining an operation of charting an auxiliarycapacitor component of a load in accordance with the embodiment of thepresent invention.

FIG. 12 is a diagram explaining an operation of an auxiliary groundleakage current in accordance with the embodiment of the presentinvention.

FIG. 13 is a diagram explaining a specific embodiment to which thepresent invention is applied.

DESCRIPTION OF SPECIFIC EMBODIMENTS

Exemplary embodiments of the present invention will be described belowin more detail with reference to the accompanying drawings. The presentinvention may, however, be embodied in different forms and should not beconstructed as limited to the embodiments set forth herein. Rather,these embodiments are provided so that this disclosure will be thoroughand complete, and will fully convey the scope of the present inventionto those skilled in the art. Throughout the disclosure, like referencenumerals refer to like parts throughout the various figures andembodiments of the present invention.

FIG. 3 is a schematic function block diagram of a ground leakage currentmeasurement apparatus in accordance with an embodiment of the presentinvention. The ground leakage current measurement apparatus 100 isconnected to an ungrounded DC power system consisting of a main electricline P and an auxiliary electric line N, and includes a main storageunit 110, a main switching unit 120, an auxiliary storage unit 130, anauxiliary switching unit 140, a measurement unit 150, and a control unit160.

In this embodiment of the present invention, ‘ground leakage currentmeasurement’ is divided into a main ground leakage current measurementoperation and an auxiliary ground leakage current measurement operationwhich are mutually exclusively performed. The concept of eachmeasurement operation may be understood from a closed loop shown in FIG.4. In FIG. 4, RG1 represents a ground resistor depending on a mainground fault (hereinafter, referred to ‘main ground resistor’), and RG2represents an auxiliary resistor depending on an auxiliary-side groundfault (hereinafter, referred to as ‘auxiliary ground resistor’).

Furthermore, a symbol A represents a closed loop of the main storageunit 110 and the main switching unit 120→the main electric line P→themain ground resistor RG1→a ground path GP→the measurement unit 150→themain storage unit 110 and the main switching unit 120, and a symbol Brepresents a closed loop of the auxiliary storage unit 130 and theauxiliary switching unit 140→the measurement unit 150→the ground pathGP→the auxiliary ground resistor RG2→the auxiliary electric line N→theauxiliary storage unit 130 and the auxiliary switching unit 140. Here,the ground path GP indicates a path passing through the ground formedbetween the measurement unit 150 and the ground resistors RG1 and RG2.The closed loops A and B are established only when a ground faultoccurs.

Referring to FIG. 3, the main storage unit 110 stores power of theelectric line, and may include a capacitor (or condenser). The powerstored in the main storage unit 110 is used for the main ground leakagecurrent measurement operation, and hereinafter referred to as ‘mainmeasurement power’.

The main switching unit 120 performs switching to supply the power ofthe electric line to the main storage unit 110 according to a controlsignal of the control unit 160 to be described below, or desirably, aswitching signal indicating ON/OFF. Furthermore, the main switching unit120 performs switching to supply the main measurement power of the mainstorage unit 110 to the main ground resistor RG1, and the mainmeasurement power is supplied to a load RL through the main electricline P to charge a main capacitor component CL1 of the load RL, forexample, a noise removal capacitor or the like.

When the main capacitor component CL1 of the load RL is charged, most ofcurrent of the main measurement power is passed toward the main groundresistor RG1, during an actual main ground leakage current measurementoperation. Ideally, all of the current may flow into the main groundresistor RG1. Therefore, it is possible to minimize the influence ofcapacitive ground leakage current caused by the main capacitor componentCL1 of the load RL and thus measure a pure resistive ground leakagecurrent caused by the main ground resistor RG1. Furthermore, as the maincapacitor component of the load is charged, it is possible to minimizethe influence of a distributed capacitor component (not illustrated)between the main electric line and the ground.

Meanwhile, the auxiliary storage unit 130 may include a capacitor orcondenser, like the main storage unit 110, and stores the power of theelectric line as auxiliary measurement power. The auxiliary measurementpower and the above-described main measurement power are stored at thesame potential as a voltage between the electric lines P and N. Forexample, when the voltage between the main and auxiliary electric linesis 48V, the main measurement power and the auxiliary measurement powerare also stored at 48V. That is, power required for the main theauxiliary ground leakage current measurement operation is provided at48V.

The auxiliary switching unit 140 performs switching to supply the powerof the electric line to the auxiliary storage unit 130 according to aswitching signal of the control unit 160, and performs switching tosupply the auxiliary measurement power stored in the auxiliary storageunit 130 to the auxiliary ground resistor RG2, and the auxiliarymeasurement power is provided to the load RL through the ground path GPto charge the auxiliary capacitor component CL2. When the auxiliarycapacitor component CL2 is charged, the current of the auxiliarymeasurement power is passed toward the auxiliary ground resistor RG2during the auxiliary ground leakage current measurement operation. Thismay be understood in the same context as the main capacitor component ischarged.

The measurement unit 150 is connected between the switching unit 120 and140 and the ground GND, forms a path to the ground resistors RG1 and RG2through the ground path GP when a ground fault occurs, and measures anauxiliary or main ground leakage current. Here, the measurement unit 150may include a function of calculating a main or auxiliary groundresistance value from the measured ground leakage current. Theembodiment of the present invention is not limited thereto, and thecontrol unit 160 may include the function of calculating a groundresistance value.

The control unit 160 controls the main and auxiliary switching units 120and 140 based on a switching signal, in order to discriminate the mainground leakage current measurement operation and the auxiliary groundleakage current measurement operation. The detailed descriptions of thefunction and operation of the control unit 160 will be made below.

FIG. 5 is a diagram illustrating a basic circuit configuration of theground leakage current measurement apparatus. The main storage unit 110has one end connected to the main electric line P. The above-describedswitching unit 120 includes a first switch 122, a second switch 124, anda third switch 126. The first switch 122 has one end connected to theauxiliary electric line N and the other end connected to the other endof the main storage unit 110, and is switched in response to a switchingsignal of the control unit 160 to store the main measurement power inthe main storage unit 110. The second switch 124 is connected betweenthe other end of the main storage unit 110 and one end of themeasurement unit 150 and passes the power of the main storage unit 110to the main ground resistor RG1 according to a switching signal. Thethird switch 126 is connected between the other end of the main storageunit 110 and the other end of the measurement unit 150, and supplies themain measurement power of the main storage unit 110 to the load throughthe main electric line P according to a switching signal, therebycharging the main capacitor component CL1 of the load.

In this specification, the switching unit may include a metal-oxidesemiconductor field-effect transistor (MOSFET) having high-voltage andhigh-current characteristics. However, the embodiment of the presentinvention is not limited thereto, and a mechanical switch which isswitched according to a switching signal (ON/OFF signal) of the controlunit may be used.

Meanwhile, the auxiliary storage unit 130 has one end connected to theauxiliary electric line N. The auxiliary switching unit 140 includes afourth switch 142, a fifth switch 144, and a sixth switch 146. Thefourth switch 142 has one end connected to the main electric line P andthe other end connected to the other end of the auxiliary storage unit130, and stores the auxiliary measurement power in the auxiliary storageunit 130 according to a switching signal of the control unit 160. Thefifth switch 144 is connected between the other end of the auxiliarystorage unit 130 and the one end of the measurement unit 150, and passesthe auxiliary measurement power of the auxiliary storage unit 110 to theauxiliary ground resistor RG2 according to a switching signal. The sixthswitch 146 is connected between the other end of the auxiliary storageunit 130 and the other end of the measurement unit 150, and supplies theauxiliary measurement power of the auxiliary storage unit 130 to theload through the ground path GP according to a switching signal, therebycharging the auxiliary capacitor component CL2 of the load.

Meanwhile, referring to FIG. 6, the ground leakage current measurementprocess in accordance with the embodiment of the present invention mayinclude: storing the main measurement power at step S110, charging themain capacitor component of the load at step S120, measuring a mainground leakage current at step S130, storing the auxiliary measurementpower at step S140, charging the auxiliary capacitor component of theload at step S150, and measuring an auxiliary ground leakage current atstep S160. Desirably, the steps S110 to S160 may be repetitivelyperformed. Hereinafter, each of the steps will be described with a tableshowing the on/off states of the respective switches.

[Storing Main Measurement Power—S110]

Referring to FIG. 7, the control unit 160 applies a switching signal(not illustrated), that is, an ON signal to the first switch 122 tocharge the main storage unit 110. The main storage unit 110 is chargedwith a voltage between the main electric line P and the auxiliaryelectric line N, and the voltage is used as the main measurement power.

TABLE 1 Switch 122 124 126 142 144 146 State ON OFF OFF OFF OFF OFF

[Charging Main Capacitor Component of Load—S120]

When the main measurement power is stored, the control unit 160 turns onthe third switch 126 through a switching signal as shown in FIG. 8, suchthat the main capacitor component CL1 of the load RL is charged with thepower of the main storage unit 110. At this time, the distributedcapacitor component between the main electric line P and the ground isalso charged. Therefore, it is possible to minimize the influence of acapacitive ground leakage current caused by the distributed capacitorcomponent and the capacitor component of the load in the next step ofmeasuring a main ground leakage current.

TABLE 2 Switch 122 124 126 142 144 146 State OFF OFF ON OFF OFF OFF

[Measuring Main Ground Leakage Current—S130]

Referring to FIG. 9, the control unit 160 turns on the second switch 124to form a path where the main measurement power of the main storage unit110 passes through the main electric line P, the main ground resistorRG1, the ground path GP, and the measurement unit 120. The measurementunit 150 measures a current flowing in the path as the main groundleakage current.

TABLE 3 Switch 122 124 126 142 144 146 State OFF ON OFF OFF OFF OFF

[Storing Auxiliary Measurement Power—S140]

Referring to FIG. 10, the control unit 160 applies a switching signal tothe fourth switch 142 to store the auxiliary measurement power in theauxiliary storage unit 130.

TABLE 4 Switch 122 124 126 142 144 146 State OFF OFF OFF ON OFF OFF

[Charging Auxiliary Capacitor Component of Load—S150]

When the auxiliary measurement power is stored, the control unit 160turns on the sixth switch 146 through a switching signal as shown inFIG. 11, such that the auxiliary measurement power of the auxiliarystorage unit 130 is supplied to the load RL through the ground path GP,thereby charging the auxiliary capacitor component CL2 of the load.Accordingly, it is possible to minimize the influence of a capacitiveleading peak current caused by the distributed capacitor component andthe capacitor component of the load in the next step of measuring anauxiliary ground leakage current.

TABLE 5 Switch 122 124 126 142 144 146 State OFF OFF OFF OFF OFF ON

[Measuring Auxiliary Ground Leakage Current—S160]

Referring to FIG. 12, the control unit 160 turns on the fifth switch 144through a switching signal and forms a path where the auxiliarymeasurement power of the auxiliary storage unit 130 pass through theground path GP, the auxiliary ground resistor RG2, the auxiliaryelectric line N, and the measurement unit 150. The measurement unit 150measures a current flowing in the path as the auxiliary ground leakagecurrent.

TABLE 6 Switch 122 124 126 142 144 146 State OFF OFF OFF OFF ON OFF

As described above, the ground leakage current measurement operation inaccordance with the embodiment of the present invention is divided intothe main ground leakage current measurement operation and the auxiliaryground leakage current measurement operation which are mutuallyexclusively performed. Accordingly, although ground faultssimultaneously occur in the main and auxiliary electric lines or groundfaults having the same ground resistance value simultaneously occur, itis possible to measure a ground leakage current. Furthermore, since theinfluences of the distributed capacitor component and the capacitorcomponent of the load between the electric line and the ground areminimized before the ground leakage current measurement, it is possibleto measure a pure resistive ground leakage current. The ground leakagecurrent measured in such a manner is used to accurately calculate themain ground resistance value or auxiliary ground resistance value.

Hereinafter, referring to FIG. 13, a specific embodiment to which theabove-described configuration of the present invention is applied willbe described. The embodiment is only an example for explaining theindustrial applicability of the present invention. Therefore, thedetailed descriptions of specific functions of additional componentswill be omitted.

Referring to FIG. 13, the ground leakage current measurement apparatus100 may further include a ZCT 101, a signal detection unit 170, adisplay unit 180, and an alarm transmission unit 190. The signaldetection unit 170, the display unit 180, and the alarm transmissionunit 190 are controlled by the control unit 160.

Specifically, the ZCT 101 is a non-contact type sensor for measuring aground leakage current of an electric line, and is installed at eachdiverging point of the electric line. The signal detection unit 170detects a ground leakage current from a signal outputted from the ZCT101. At this time, the ground leakage current is different from theabove-described main and auxiliary ground leakage currents. The groundleakage current detected by the signal detection unit 170 is a currentwhich occurs in a diverging point where the ZCT 101 is installed, thatis, on a diverging electric line.

The display unit 180 visually outputs the overall control state of theapparatus 100, the ground leakage current value and the groundresistance value measured by the measurement unit 150, and the groundleakage current value detected by the signal detection unit 170. Thealarm transmission unit 190 is an interface for transmitting relatedinformation during a ground fault to a remote manager terminal.

The ground leakage current detection apparatus 100 in accordance withthe specific embodiment of the present invention has an advantage inthat a separate signal generator does not need to be used to transmit apredetermined supervisory signal to an electric line, unlike theconventional ground leakage current detection apparatus. That is becausethe main or auxiliary measurement power which is momentarily supplied tothe electric line is used as a supervisory signal at the above-describedstep S130 or S160 of measuring the main or auxiliary ground leakagecurrent. That is, the ground leakage current measurement apparatus 100in accordance with the embodiment of the present invention also includesa function of a signal generator which is driven by its own power.

While the present invention has been described with respect to thespecific embodiments, it will be apparent to those skilled in the artthat various changes and modifications may be made without departingfrom the spirit and scope of the invention as defined in the followingclaims.

1. A ground leakage current measurement apparatus in an ungrounded DCpower system including main and auxiliary electric lines, the groundleakage current measurement apparatus comprising: a switching unitconfigured to perform switching to supply measurement power to a mainground resistor and an auxiliary ground resistor by using power of theelectric lines; a measurement unit connected between the switching unitand the ground and configured to measure at least one of main andauxiliary ground leakage currents; and a control unit configured tocontrol the switching unit to discriminate a main ground leakage currentoperation and an auxiliary ground leakage current operation of themeasurement unit.
 2. The ground leakage current measurement apparatus ofclaim 1, wherein the switching unit comprises: a main storage sectionconfigured to store main measurement power from the power of theelectric lines; a main switching section configured to perform switchingto supply the main measurement power to the main ground resistor; anauxiliary storage section configured to store auxiliary measurementpower from the power of the electric lines; and an auxiliary switchingsection configured to perform switching to supply the auxiliarymeasurement power to the auxiliary ground resistor.
 3. The groundleakage current measurement apparatus of claim 2, wherein the controlunit controls the main switching section to supply the main measurementpower to a load before the main ground leakage current measurementoperation of the measurement unit, thereby charging a main capacitorcomponent of the load.
 4. The ground leakage current measurementapparatus of claim 2, wherein the control unit controls the auxiliaryswitching section to supply the auxiliary measurement power to a loadbefore the auxiliary ground leakage current measurement operation of themeasurement unit, thereby charging an auxiliary capacitor component ofthe load.
 5. The ground leakage current measurement apparatus of claim2, wherein the main switching section comprises: a first switch havingone end connected to the auxiliary electric line and the other endconnected to one end of the main storage section connected to the mainelectric line; and a second switch connected between the one end of themain storage section and one end of the measurement unit, and groundedthrough the measurement unit.
 6. The ground leakage current measurementapparatus of claim 5, further comprising a third switch connected to theone end of the main storage section and the other end of the measurementunit.
 7. The ground leakage current measurement apparatus of claim 2,wherein the auxiliary switching section comprises: a fourth switchhaving one end connected to the main electric line and the other endconnected to one end of the main storage section connected to theauxiliary electric line; and a fifth switch connected between one end ofthe auxiliary charging section and one end of the measurement unit andgrounded through the measurement unit.
 8. The ground leakage currentmeasurement apparatus of claim 7, further comprising a sixth switchconnected to the one end of the auxiliary storage section and the otherend of the measurement unit.
 9. A ground leakage current measurementmethod in an ungrounded DC power system including main and auxiliaryelectric lines and a control unit, the ground leakage currentmeasurement method comprising: supplying measurement power to a main orauxiliary ground resistor by using power of the electric lines, undercontrol of the control unit; and measuring at least one of main andauxiliary ground leakage currents passing through the ground resistor,under control of the control unit which discriminates a measurementoperation.
 10. The ground leakage current measurement method of claim 9,wherein the supplying of the measurement power comprises: storing mainmeasurement power by using the power of the electric lines; andsupplying the main measurement power to the main ground resistor. 11.The ground leakage current measurement method of claim 9, wherein thesupplying of the measurement power comprises: storing auxiliarymeasurement power by using the power of the electric lines; andsupplying the auxiliary measurement power to the auxiliary groundresistor.
 12. The ground leakage current measurement method of claim 10,further comprising supplying the main measurement power to a load tocharge a main capacitor component of the load, before supplying the mainmeasurement power to the main ground resistor.
 13. The ground leakagecurrent measurement method of claim 11, further comprising supplying theauxiliary measurement power to a load to charge an auxiliary capacitorcomponent of the load, before supplying the auxiliary measurement powerto the auxiliary ground resistor.